Optical systems have been developed that are used to make optical measurements. For example, a spectrophotometer is an optical system than can be used to measure the level of transmission or absorption of a sample material with respect to a number of different wavelengths of radiation. A spectrophotometer has a radiation source that transmits radiation along a path of travel to a radiation detector. During operational use, the sample under test is positioned optically between the source and the detector, along the path of travel. Radiation from the source that is traveling along the path of travel must pass through the sample, and the detector measures the intensity of received radiation, which represents the amount of radiation that is able to pass through the sample. The accuracy of optical measurements provided by such a system depends on the accuracy of the calibration of the system.
It is relatively simple to calibrate a spectrophometer for a transmissivity of 0% and/or a transmissivity of 100%. In particular, it is easy to completely block the radiation beam, or to leave it completely unblocked. However, radiation detectors are typically nonlinear, and in fact there may be differences in the nonlinearity of equivalent detectors that in theory should be identical. Consequently, calibrating for only 0% and/or 100% is not sufficient. It is desirable to perform calibration for one or more different levels of transmissivity that are between 0% and 100%. This can improve the accuracy of the calibration, for example by an average of a factor of ten.
A related consideration is that radiation detectors are not always spatially uniform. For example radiation impinging on one portion of the detector may produce a different measurement than if that same radiation were to impinge on a different portion of the same detector.
To calibrate for a level of transmissivity between 0% and 100%, a traditional approach is to insert a stationary optical reference (or several successive stationary references) between the source and detector. Each such optical reference has a known transmissivity. One known type of optical reference is a filter with a known transmissivity, typically a neutral density filter. However, filters of this type work only for particular wavelength ranges. Further, materials in the filter may gradually deteriorate and change performance, due to handling, exposure and/or aging. Care must be taken to avoid abrading, scratching or otherwise altering the filter. Moreover, contaminates from the air can accumulate on the filter, altering performance. Cleaning the surface of the filter to remove contaminates may alter the performance of the filter.
A different type of known optical reference is made from a material that is well characterized. For example, the optical reference may be a piece of calcium fluoride (CaF2). This type of reference can be more stable than a neutral density filter, but is still subject to some of the same problems. Further, only a limited selection of transmissivity levels may be available. For example, in the visible spectrum, there are very few materials having a transmissivity in the 0% to 70% range.
Thus, although existing optical references and calibration techniques have been generally adequate for their intended purposes, they have not been satisfactory in all respects. For example, existing optical references used for calibration are not always durable, stable and highly accurate, and cannot always be obtained for every desired level of transmissivity.